Beat the cream with the vanilla sugar and the cream stiffener until stiff. Sweeten the yogurt with the sugar to taste, then fold in the cream.
Put two spoons of the cream and yoghurt cream in a baking dish, then cover with a layer of biscuits. Pour some cocoa over it, then another layer of the cream. Sprinkle the frozen fruit on top, then another layer of shortbread, cocoa and cream. On top you can then sprinkle a few fruits or sprinkle with cocoa or chocolate shavings.
The tiramisu tastes best when it can last a night.